This invention relates to automatic soldering machines for soldering components on printed circuit boards.
Some PCB's do not have through holes and these are known as surface mount devices (SMD's). A preferred soldering technique for SMD's is infrared (IR) reflow in which solder paste on the PCB's is caused to reflow upon the application of heat. More particularly, heating takes place in separately controlled zones within the machine permitting precise temperature profiling. The boards pass by wire mesh conveyor first into a preheat zone which gently heats the solder paste to drive off volatile solvents and avoid thermal shock to the PCB's. After preheating, the PCB's pass into the main heating zones where they are raised to reflow temperature. On exiting from the heating zones the boards are cooled, typically by blowing air up through the conveyor mesh, to solidify the solder joints.
In a development of this technique, automatic soldering machines have been designed in which the IR heating is supplemented with forced convection. In one such machine marketed by the assignee of the present application, blowers are provided above a top row of heaters which are perforated so that hot air is blown on to the PCB's .
So as to reduce or eliminate problems caused by oxidation of the solder paste it has been proposed that, instead of air, an inert gas such as nitrogen should be used for the atmosphere inside the heating zones and that forced convection would involve blowing the inert gas rather than air.